This kind of active security sensor system has been well known to those skilled in the art, in which a light projector of the infrared light transmitting unit and a light receiver of the infrared light receiving unit, positioned on respective opposite end portions of a linear warning area in optically aligned relation to each other. The warning area is generally set to have a linear coverage from a close range to a long distance of, for example, a few hundred meters and, as the warning area expands over the long distance, difficulties have been encountered in aligning respective optical axes of the light projector and the light receiver with each other. In view of this, the prior art security sensor system makes use of a sighting device, such as shown in FIGS. 13A and 13B of the accompanying drawings, in each of the light projector and the light receiver so that following installation of the sensor system and/or during the maintenance and servicing the optical alignment or sighting is carried out relying on the sighting device employed in each of the light projector and receiver.
The discussion over the prior art believed to be pertinent to the present invention will be made with reference to FIGS. 13A and 13B, both of which illustrate a schematic transverse sectional view of the prior art sighting device. The sighting device 50 includes a casing having a pair of viewing windows 51, 51 a pair of objective windows 52, 52, both windows defined in the wall of the casing, and a pair of sighting mirrors 53, 53 disposed on respective sighting axes 56, 56, each extends between the viewing window 51 and the objective window 52. As a matter of course, the illustrated sighting device 50 is employed in conjunction with each of the infrared light projector and the infrared light receiver, and, therefore, the image sighted through the sighting device employed in the infrared light projector is that of the infrared light receiver whereas the image sighted through the sighting device employed in the infrared light receiver is that of the infrared light projector. Accordingly, if the respective optical axes of the light projector and receiver are aligned with each other, the sighting axes naturally align with those optical axes, respectively, indicating that the infrared light projector and receiver are properly and accurately positioned relative to each other.
In accomplishing the optical alignment with the use of the prior art sighting device specifically shown in FIG. 13A, while the operator, who is assigned to work on the optical alignment using one of his or her eyes 60, looks into through either one of the left and right viewing windows 51, a dial or an adjustment screw has to be manipulated to move the infrared light projector or receiver in a direction up and down and left and right to bring the image of the infrared light receiver or projector, cast on the sighting mirror 53, to the geometric center of the objective window 52. It has, however, been found that if as shown in FIG. 13B the operator performing the optical alignment looks into through the viewing window 51 in a wrong glancing angle, that is, from a slantwise direction, not from front, the viewing axis 55, through which the operator views the image on the sighting mirror 53, is displaced relative to the sighting axis 56 (i.e., the axis extending from the light projector to the light receiver or from the light receiver to the light projector, which are opposed each other). If the optical alignment is carried out under the condition, in which the viewing axis 55 and the sighting axis 56 are displaced relative to each other in this way, no optical axis is accurately adjusted. For this reason, the operator performing the optical alignment of the security sensor system must move his eye 60 to align the viewing axis 55 from his eye 60 with the sighting axis 56 as exactly as possible.
In order for the eye of the operator, then looking into through the viewing window 51, to be properly positioned to align the viewing axis with the sighting axis, the security sensor system of a type utilizing, in each of the light projector and the light receiver, a tubular viewing scope extending from the corresponding viewing window 51 in a direction outwardly of the sighting device 50 is currently available in the art. It has, however, been found that if the viewing window 51 is made large in size in order to secure a relatively large field of view, the tubular viewing scope will project a substantial length outwardly from the sighting device, thus rendering the security sensor system, equipped with the tubular viewing scopes, to be difficult in handling and also to have an appearance unpleasant to look.
The JP Laid-open Patent Publication No. 08-248479 discloses a camera viewfinder, not a security sensor in any way what so ever, of a structure in which a target marking indicative of the geometric center of the field of view appearing in the viewfinder is formed in an objective lens and a microlens is incorporated in the eyepiece lens. According to this publication, the target marking indicative of the geometric center of the field of view of the viewfinder is clearly observed through the microlens together with an image of an object to be viewed.
It has, however, been found that the teachings of the above mentioned publication are in no way applicable to the present invention because of the following reason. Specifically, the target marking employed in the camera viewfinder is used for the purpose of aligning the image of the object to be viewed with the geometric center of the field of view of the viewfinder. Therefore, the necessity of adjusting the viewing axis of a person looking through the microlens by positioning his or her eye squarely with the microlens is in no way recognized in the camera viewfinder disclosed in the above mentioned publication.